(1) Technical Field of the Invention
This invention relates generally to the repair of tubes. In particular, this invention is directed to methods of and apparatus for use in the repair of tubes in a heat exchanger such as, for example, a nuclear steam generator.
Steam generators are large heat exchangers which are commonly employed in electric power generating installations. In the operation of nuclear power reactors, a steam generator is interposed between the nuclear reactor vessel and the turbine generator set. Hot water from the nuclear reactor is passed through the tubes of the steam generator for generating steam on the shell or secondary side of the generator.
Steam generators typically contain thousands of tubes. Leakage in some of the tubes due to corrosion or mechanical failure is a common problem in steam generators. In fossil fuel plants some degree of leakage can be tolerated. However, in nuclear plants even minor leakage can provide a path for the transfer of radioactive contamination to other components of the power plant system. Thus, measures must be taken during the periodic plant outages to repair the leaking tubes.
All steam generators are designed with excess thermal exchange capacity. Conventional tube repair is generally approached by either plugging a tube to remove the tube from service or by securing a sleeve in the tube to enable the primary fluid to by-pass the deteriorated portion of the tube. The excess thermal exchange capacity which has been specially incorporated into the steam generator allows a fraction of the tubes to be sealed off without compromising the overall effectiveness of the exchanger.
In nuclear plants radioactive contamination of the steam generator by carry-over from the reactor fuel system limits the time that repair personnel may spend in the steam generator enclosure where the repair work is to be performed. The environmental exposure constraints imposed on workmen entering a radioactive steam generator tend to mitigate against conventional repair methods and repair assemblies which require welding, brazing and other time intensive tasks. Moreover, in previously employed tube repair methods and assemblies which provide for efficient tube repair, the removal of welded assemblies and known plug assemblies have proven problematical and expensive.
It is accordingly a principal aim of the present invention to provide a new and improved steam generator tube repair assembly which overcomes many of the noted deficiencies of conventional tube repair methods and hardware.
(2) Description of the Prior Art
U.S. Pat. No. 3,900,939 discloses a method for plugging the ends of defective tubes of heat exchangers. The patented method employs a plug fabricated from a metallic alloy having a mechanical memory characteristic whereby the plug can be physically deformed into a reduced diameter for insertion into the tube end and thereafter induced to assume its original diameter to secure the plug within the tube. U.S. patent application Ser. No. 542,307 filed Oct. 14, 1983, now abandoned and assigned to the assignee of the present invention discloses a tube repair insert for securing a repair assembly in position within a tube. The repair assembly includes a cylindrical shell having an outer diameter substantially equal to the tube inner diameter. The repair assembly also includes a alloy ring having a shape memory and an outer diameter substantially equal to the shell inner diameter. The shape memory ring has a temperature threshold below the ambient tube operating temperature. The shape memory ring is maintained in the contracted condition during insertion of the ring within the shell. A leak-tight joint is formed between the expanding shell and the tube at the repair assembly location when the alloy ring reaches the threshold temperature. The alloy ring expands typically on the order of 3-5 percent.
U.S. Pat. No. 4,469,357 discloses a composite coupling device comprising two members. The first member is a heat recoverable driving member and the second is a metallic member associated with the first member in a manner wherein upon heat recovery of the driver, the second member is dimensionally transformed for engaging a cylindrical substrate. The driving member comprises a tubular heat recoverable compression sleeve having a generally uniform interior surface. The second member comprises a tubular insert snugly and concentrically disposed within the compression sleeve. Upon heat recovery of the compression sleeve the insert is compressed about the substrate. The driving member may also be positioned internally of the second member so that expansive heat recovery of the driving member expands the second member to engage the substrate.
U.S. Pat. No. 3,525,365 discloses an expansion plug for sealing passages comprising an expanding plug member and a locking member having complementary frustoconical surfaces which cooperate to provide a sealing engagement upon outward expansion of the plug.